This invention relates to apparatus and methods for shaping tubes and, in particular, to shaping glass tubes intended for use in the manufacture of optic fibers and for doing so semi-automatically or automatically.
One process for the manufacture of optical fibers is referred to as the modified chemical vapor deposition (MCVD) process in which the internal wall of a glass cylinder (also referred to herein as the xe2x80x9cstarter tubexe2x80x9d or xe2x80x9cpreform tubexe2x80x9d) is coated with uniform layers of reactants and gas vapors to form the rod from which the optic fibers are extruded. To ensure the proper and uniform flow of gases within the starter tube, it is desirable to join, or fuse, the starter tube to an exhaust tube prior to initiating the actual fiber optic manufacturing process.
For optimum results the exhaust tube should provide smooth, continuous flow for the gases escaping from the starter tube to, and through, the exhaust tube. To accomplish this result, the exhaust tube must generally have a larger diameter than the starter tube. However, at their interface and mating ends the exhaust tube must mesh smoothly and continuously with the starter tube and must have a profile which aids in the smooth flow of gases out of the starter tube.
It is also noted that each exhaust tube is intended to be joined (or fused) to a starter tube and that the two xe2x80x9ccombinedxe2x80x9d tubes are then operated as a unit. Typically, the combined tubes are mounted in an apparatus in which they are made to rotate for many hours while gases and reactants are being injected into the starter tube under intense heat conditions, for forming uniform layers of depositions for the optic fibers. To ensure the formation of consistently uniform layers, it is important that the two tubes be aligned very accurately; (i.e., have a common center line) throughout the process.
In presently known systems the end of an exhaust tube designed to mate with a starter tube is shaped manually using a graphite forming tool, or like manual equipment. This process is an xe2x80x9cartxe2x80x9d dependent on the skills and techniques of the artisan shaping the mating end and interface of the exhaust tube. This is undesirable and problematic because tubes shaped manually have little uniformity and dimensional reproducibility. As a result, numerous defective exhaust tubes are produced. Equally problematic is that, even when an exhaust tube is not defective, the mating of an exhaust tube with a starter tube is subject to alignment problems.
Thus, there is a dual problem of uniformly shaping and contouring the end of an exhaust tube so that the starter tube (at its mating output) meshes smoothly with the exhaust tube at their common interface and of aligning the two tubes accurately. Clearly, the requirements placed on the manufacture of fiber optics is very demanding and very costly. It may take many hours to produce a fiber optic xe2x80x9cpreformxe2x80x9d. Even very small defects, because of their cumulative effect, may result in the loss of much material, energy and time, at great cost to the manufacturer.
Another problem is that in accordance with the prior art when the starter and exhaust tubes are to be joined or fused together, they are cantilevered from spindle chucks. An operator must support the cantilevered tubes by manual means or through the use of a xe2x80x9cjackxe2x80x9d device and raise or lower the rotating starter and exhaust tubes to achieve co-axial alignment with the end of the exhaust tube. Heat is applied at the support point of the starter tube to stress relieve the starter tube as the operator applies a relocating force to the starter and exhaust tubes. The quality of the co-axiality between the two tubes in this butt-splice technique is a function of the operator""s skill. This is undesirable because the quality of the results is unpredictable.
Applicants"" invention is aimed at reducing the problems associated with the shaping of a first tube (e.g., an exhaust tube) which must interface and mate with a second tube (e.g., a starter tube) and ensuring the proper alignment of the two tubes.
In contrast to the prior art technique, the exhaust and starter tubes have a self centering profile, or xe2x80x9cnestingxe2x80x9d feature. The two tubes are positioned by physical contact with features formed into the tubes themselves. These features are formed on the contact (mating) faces of the tubes. In one embodiment of the invention, the ends of the exhaust tube are molded and shaped to assume a certain configuration, such as a lip, whereby the starter tube xe2x80x9clocksxe2x80x9d onto the exhaust tube.
One aspect of applicants"" invention includes a mold for shaping the opening of a selected end region of a first, hollow, generally cylindrical tube to enable a second tube, of smaller diameter than the first tube, to be inserted xe2x80x9csnugglyxe2x80x9d a first distance within opening at the selected end of the first tube. In a particular embodiment, the mold includes two, complementary elongated side pieces having inner surfaces for, when joined, encircling the periphery of the first tube along a portion of its selected end region; and an end cap with a cylindrical stub for insertion within the opening of the first tube at its selected end. The two elongated side pieces and the end cap stub are applied to the first tube when the first tube is in a malleable state. The pieces of the mold cause the inner diameter of the tube for a first distance from the end of the tube to have a first value (d2) and, at the first distance, to then decrease to a second value (d1) and to then gradually and smoothly increase to a third value (dt2). In an embodiment of the invention, the tube is a first tube for use as an exhaust tube. The end region and opening of the exhaust tube are shaped to enable a second tube for use as a starter tube, to be snuggly inserted into the opening of the first tube with the two tubes being self centered and being more easily aligned along a common center line.